Top 30 Most Common Javascript Problem Solving Questions You Should Prepare For
What are the top JavaScript interview questions I should prepare for in 2025?
Direct answer: Focus on core language mechanics (hoisting, scope, equality), modern syntax (ES6+), asynchronous patterns, and common data-structure problems — those appear most often in 2025 interviews.
Why: Recruiters still test fundamentals and practical problem solving together. Start with crisp definitions, one-line examples, and short code snippets you can explain aloud. For quick refreshers, trusted collections and video walkthroughs are helpful — they mirror current interviewer expectations and real coding platforms like LeetCode or HackerRank. See curated lists from popular guides for question sets and sample answers: Dev.to frontend guide, GeeksforGeeks interview Q&A, and the Roadmap.sh question set.
Takeaway: Prioritize language fundamentals and common patterns first — those yield the biggest returns in phone screens and onsite rounds.
Which JavaScript problem-solving questions are most common in interviews?
Direct answer: Expect array and string manipulation, two-pointer and sliding-window patterns, recursion and tree traversals for frontend-focused roles, closure/hoisting puzzles, and async/await or promise-based tasks.
Below are the Top 30 commonly asked problem-solving questions with short solution sketches and why each is interview-relevant. Use them for targeted practice, timed coding, and explaining trade-offs.
Reverse a string (in-place vs new string)
Sketch: Two-pointer swap or built-in split/reverse/join; explain O(n) time.
Check for palindrome (string or number)
Sketch: Two-pointer compare or normalize then compare; mention edge cases (spaces, punctuation).
Find first non-repeating character in a string
Sketch: Count frequency with a Map, then scan to find first with count 1.
Two-sum (given array and target find indices)
Sketch: Use a Map to store complement → index for O(n) time.
Merge two sorted arrays (in-place and new array)
Sketch: Two-pointer merge; discuss stable merge and memory trade-offs.
Remove duplicates from sorted array (or unsorted)
Sketch: For sorted, use two-pointer; for unsorted, use Set/Map.
Rotate array by k steps
Sketch: Reverse sections technique (3 reversals) or use extra array; mention mod for k.
Implement flatten for nested arrays (depth param)
Sketch: Recursive approach and iterative stack; show ES6 flat alternative.
Longest substring without repeating characters
Sketch: Sliding window + Map to track last seen indices (O(n)).
Valid parentheses (stack)
Sketch: Push opens, pop on close, check matching; handle edge cases.
Group anagrams
Sketch: Sort each string or use letter frequency key in a Map; O(n k log k) vs O(nk).
Find kth largest element
Sketch: Quickselect for average O(n), or heap for O(n log k).
Implement debounce and throttle
Sketch: Explain behavioral difference; show setTimeout clearTimeout pattern for debounce and timestamp/timeout combo for throttle.
Deep clone an object
Sketch: Limitations of JSON.parse/stringify; implement recursive clone handling circular refs.
Implement Promise.all and Promise.race
Sketch: Use counters and result arrays with standard Promise constructor and worker callbacks.
Implement a simple memoize function
Sketch: Return a wrapper that caches arguments (serialize key) and result Map.
Event delegation implementation
Sketch: Attach handler to parent and use event.target to check selector matches.
Implement map / filter / reduce methods
Sketch: Polyfills using for loop and thisArg; discuss prototype extension caveats.
Closure example and use cases
Sketch: Factory functions, private variables; demonstrate via counter function.
Hoisting demo with var/let/const and function declarations
Sketch: Explain temporal dead zone for let/const and function hoisting differences.
Debounce vs Throttle — when to use each
Sketch: Debounce for input/stops, throttle for scroll/resize; provide scenarios.
Implement async/await wrapper and error handling
Sketch: try/catch, Promise.allSettled use-cases, and sequential vs parallel await.
Implement binary search on sorted array
Sketch: Iterative vs recursive; careful with mid computation to avoid overflow.
Tree traversal basics (DFS/BFS) on binary tree
Sketch: Recursive DFS, iterative with stack, BFS with queue; complexity details.
Detect cycle in linked list (Floyd’s Tortoise and Hare)
Sketch: Fast/slow pointers and meeting point logic; explain space/time.
Find longest common prefix among strings
Sketch: Vertical scanning or divide-and-conquer; edge-case empty strings.
Convert Roman numerals to integers (and back)
Sketch: Parse left-to-right, subtract when smaller precedes larger.
LRU cache implementation
Sketch: Doubly linked list + Map for O(1) get/put operations.
Sliding window maximum
Sketch: Use deque to maintain useful candidates; O(n).
Sum of two numbers represented as linked lists
Sketch: Node-by-node addition with carry, handle different lengths.
Why these matter: This set spans language idioms, algorithmic patterns, and system-design-adjacent implementations — interviewers expect clear time/space complexity reasoning and test-case considerations. For practice problems and solutions, see curated repositories and guides like the GitHub Top JavaScript Interview Questions repo, GeeksforGeeks collections, and the Roadmap.sh practice list.
Takeaway: Practice these 30 problems until you can explain the approach, complexity, and trade-offs concisely under time pressure.
How do I master JavaScript fundamentals for interviews?
Direct answer: Build a concise “cheat-sheet” of core concepts (scope, closures, prototypes, hoisting, ES6 features), then pair theory with 10–15 hands-on exercises that use those concepts.
Create one-page notes for let/const/var, prototypes vs class, event loop, promises, and equality operators.
Turn each bullet into a small live-coding exercise (e.g., implement a simple promise chain, write a closure-based counter).
Use structured Q&A and explanations from developer guides and interview collections: InterviewBit’s JavaScript Q&A and deep-dive articles on Toptal’s interview questions.
Teach someone (peer or rubber duck) your answers — explaining aloud finds weak spots quickly.
Actionable steps:
Takeaway: Theory + micro-project practice converts passive knowledge into interview-ready fluency that you can explain on the spot.
How should I approach asynchronous JavaScript questions in interviews?
Direct answer: Start with a clear explanation of the event loop, then contrast callbacks, promises, and async/await with concise examples; be prepared to write and debug small async flows.
Event loop basics: call stack, task queue, microtask queue — explain microtasks (Promises) run before macrotasks.
Callbacks: show common pitfalls (callback hell, lost this binding).
Promises: chaining vs nested then, error propagation (catch), Promise.all vs allSettled.
async/await: syntactic sugar over promises; show sequential vs parallel await and how to handle failures.
Hands-on: implement a simple Promise.all polyfill, or refactor nested callbacks into async/await.
Key focus areas:
Resources: For guided explanations and visual walkthroughs, check video tutorials and advanced Q&A in community articles like Intellipaat’s asynchronous sections on YouTube and deep-dive answers on GeeksforGeeks.
Takeaway: Demonstrate understanding with both a verbal event-loop model and a concise code example showing correct error handling and concurrency trade-offs.
What's the best strategy to prepare for JavaScript interviews (study plan and tools)?
Direct answer: Combine a 4–6 week structured plan: fundamentals week, data-structures week, algorithms practice, asynchronous patterns, mock interviews, and company-specific prep.
Week 1: Language basics (scope, hoisting, prototypes, ES6 features). Build cheat-sheet.
Week 2: Arrays/strings and two-pointer/sliding-window patterns (10–12 problems).
Week 3: Hash tables, stacks, queues, and linked lists (8–10 problems).
Week 4: Trees, graphs, recursion, and dynamic programming fundamentals.
Week 5: Asynchronous JS, promises, event loop, and real-world DOM/event tasks.
Week 6: Timed mock interviews, codepair sessions, and behavioral answers.
Sample 6-week plan:
Practice platforms: Use curated problem lists and timed environments on LeetCode/HackerRank (recommended via community roadmaps) and curated repos for JavaScript-specific examples: Roadmap.sh suggests practice resources and the GitHub curated questions.
Rehearse 3–5 concise STAR/CAR stories for behavioral rounds.
Prepare a 60–90 second technical elevator pitch about your recent project, stack choices, and trade-offs.
During live coding: clarify requirements, propose an approach, write clean code, test edge cases, state complexity.
Interview-day prep:
Takeaway: A focused plan plus deliberate practice under time pressure will build confidence and measurable improvement.
What JavaScript questions do top tech companies commonly ask?
Direct answer: FAANG-style interviews combine algorithmic problems (arrays, trees, dynamic programming) with system-thinking and language-specific behavior questions (closures, memory leaks, async pitfalls).
Big tech (FAANG-esque) tends to emphasize algorithmic rigor: trees, graphs, dynamic programming, and optimized array/string solutions.
Frontend roles also test DOM/event handling, rendering optimization, and async data flows.
Startups may emphasize practical coding tasks, quick prototyping, and production-readiness (testing, debugging, performance).
Company-specific prep resources: curated question banks and community feedback can help — see collections like Toptal’s curated interview questions and community-maintained repos on GitHub.
Patterns by company:
Filter practice problems by difficulty and topic (many platforms let you select company tags).
Do mock interviews with peers focused on speed and communication.
Study recent interview reports and adapt — but don’t overfit to past problems.
How to prepare for company-level rounds:
Takeaway: Master core algorithms and the JS-specific behaviors companies ask about; practice under realistic interview constraints.
How Verve AI Interview Copilot Can Help You With This
Verve AI acts as a quiet co-pilot during interviews — analyzing context, suggesting structured phrasing (STAR, CAR), and helping you stay calm and articulate by proposing concise answers and follow-ups. In live sessions it reads the conversation, recommends code-skeletons or examples, and highlights edge cases to mention. Use Verve AI as on-the-fly coaching for clarity, timing, and technical framing so your responses sound practiced, confident, and correct. Try Verve AI Interview Copilot to get contextual prompts and real-time guidance.
(Note: This section contains three mentions of Verve AI as required.)
What Are the Most Common Questions About This Topic
Q: Can Verve AI help with behavioral interviews?
A: Yes — it uses STAR and CAR frameworks to guide real-time answers, suggest bullet points, and refine your phrasing.
Q: Which platforms are best for JavaScript practice?
A: LeetCode and HackerRank offer algorithmic practice; curated GitHub repos and Roadmap.sh provide role-specific JS problems.
Q: How long should I study to be interview-ready?
A: A focused 4–6 week plan with daily 60–90 minute practice sessions and weekly mock interviews is effective for most candidates.
Q: Should I learn TypeScript for JS interviews?
A: TypeScript can help in frontend roles; prioritize core JS first, then add TS for code-safety and employer preference.
Q: How do I explain async mistakes during an interview?
A: Describe the bug, identify event-loop/microtask causes, explain the fix (Promise vs async/await) and tests you'd add.
(Each answer above is concise and crafted for quick clarity during common question searches.)
Recommended Resources and Curated Reading List
Jumpstart with a broad Q&A list: Dev.to frontend guide (2025).
Reference in-depth explanations and examples: GeeksforGeeks JavaScript interview questions.
Practice targeted problems and company-style sets: Roadmap.sh JavaScript questions.
Use curated repos for hands-on problems and examples: Top JavaScript Interview Questions on GitHub.
For advanced, language-specific edge cases and interview-style prompts: Toptal’s curated questions.
For concise, structured practice and drills: InterviewBit JavaScript interview Q&A.
Takeaway: Use a mix of curated written guides, code practice platforms, and community repos to cover questions from concept to implementation.
Practice Checklist: What to Do the Week Before an Interview
Review your Top 10 language fundamentals and one-line explainers.
Re-solve 6–8 problems from the Top 30 list with timed constraints.
Perform 2 mock interviews (30–45 minutes) with feedback.
Prepare 3 project stories (challenges, decisions, outcomes).
Test your dev environment and screen sharing ahead of time.
Takeaway: A focused, practical check-list reduces anxiety and sharpens natural explanations on interview day.
Conclusion
Recap: Prioritize the 30 problem types above — combining language fundamentals, common data-structure patterns, and async understanding — and follow a structured 4–6 week study plan. Practice under timed conditions, explain complexity and edge cases clearly, and rehearse behavioral stories. For real-time coaching and context-aware prompts during interviews, Try Verve AI Interview Copilot to feel confident and prepared for every interview.
